An electroluminescence (EL) phosphors are voltage excitation type phosphors, and a dispersion-type EL device, in which the phosphor powder is sandwiched between electrodes to provide a luminescent element, and a thin film type EL device are known. The dispersion-type EL device generally has a structure sandwiching a dispersion of a phosphor powder in a high dielectric constant binder between two electrodes at least one of which is transparent. It emits light on applying an alternating electric field between the electrodes. The light emitting devices fabricated by using an EL phosphor powder could be made to have a thickness of several millimeters or smaller and have many advantages, such as emitting surface emission, reduced heat generation, and good luminescence efficiency. It is therefore expected to be applicable as road signs, interior and exterior lights, light sources of flat panel displays, such as LCDs, light sources for large area advertisements, and the like.
Nevertheless, light emitting devices fabricated using a phosphor powder have disadvantages of lower luminescence brightness, shorter emission lifetime, and uneven emission as compared with those predicated on other principles. Various improvements have hitherto been added to overcome these disadvantages. In particular, commercially available phosphor powders generally have a large average particle size in the range of from 20 to 30 μm. Smaller and monodispersed phosphor particles have therefore been demanded.
It is known that luminescence brightness may be increased by using small phosphor particles as proposed in patent JP-A-2002-235080 and JP-A-2004-265866. In the fabrication of an EL device, using small phosphor particles increases the number of phosphor particles per unit volume of the light emitting layer, thereby binging about improved brightness. In fact, however, luminescence brightness does not increase in proportion to the number of the phosphor particles. Moreover, an El device using small phosphor particles suffers from rapid deterioration in brightness.
JP-A-2007-326981 describes a method for obtaining uniform light emission by specifying the particle size distribution in terms of volume based cumulative distribution. Improved dispersibility is obtained by using phosphor particles the size of which is large but specified in terms of cumulative distribution, whereby the problem of uneven luminescence observed in applications to lighting equipment utilizing photoluminescence represented by a white LED is eliminated.